Scientists Reveal a New Way Viruses Cause Cells to Self-Destruct

UPTON, NY -- Scientists at the U.S. Department of Energy's Brookhaven
National Laboratory and their collaborators have discovered that some
viruses can use the most abundant protein in the cells they are infecting
to destroy the cells and allow new viruses to escape to infect others. The
findings, described in the November 29, 2002, issue of the Journal of
Biological Chemistry, build upon earlier Brookhaven research on how
virus particles become infectious (see
related
story) and may lead to the design of more effective antiviral
remedies.

"This
is a new and philosophically interesting way for a virus to escape from
cells," said Brookhaven biologist Walter Mangel, a coauthor on the paper.
"In essence, a protein in the infected cells can serve as the seed of the
cellsí own destruction."

Mangel's group has previously shown that adenovirus -- a virus that
causes respiratory and gastrointestinal infections and also conjunctivitis
-- produces a protein-cleaving enzyme, or protease, to complete the
maturation of newly synthesized virus particles. Similar to the way
supportive scaffolding is removed after the completion of a construction
project, this protease cleaves, or cuts out, viral "construction"
proteins, leaving infectious virus particles behind.

This viral protease is produced in the cytoplasm in an inactive form,
and must migrate to the nucleus to become activated in newly synthesized
viral particles by two viral cofactors. Once activated, it can cleave
several viral proteins to complete the viral maturation process. There
were no indications that the protease could be activated in the cellís
cytoplasm.

When Mangel presented this research at a seminar at Princeton
University, Clarence Schutt, a Princeton chemistry professor, pointed out
that the amino acid sequence of one of the viral cofactors was
dramatically similar to the sequence of actin, a cytoplasmic protein that
gives shape and structure to cells. Mangel wondered if actin could
activate the protease, and took some from Schutt's lab back to Brookhaven
to do the experiment.

This ribbon diagram is a theoretical
representation of the structure of adenovirus protease (red) bound
to the cytoskeleton protein actin (green). The blue, green, and
yellow balls show the location of the active site of the adenovirus
protease.

The result: Incubating actin and the adenovirus protease increased the
cleaving ability of the protease, just like the viral cofactor did,
allowing the actin-protease complex to cleave actin itself, as well as
other cellular skeleton proteins.

"When actin and other cytoskeleton proteins are destroyed," Mangel
explained, "a cell loses its shape and eventually breaks open, allowing
the newly synthesized virus particles to escape and infect other cells."

The same two cells were photographed to show the
location of the adenovirus protease (labeled with a green
fluorescent molecule) and the cytoskeleton protein cytokeratin 18
(labeled with a red fluorescent molecule). The adenovirus protease
is known to cleave cytokeratin 18 (as well as other cytoskeletal
proteins), and this experiment shows they are located at the same
sites within the cells.

While other viruses are known to cleave actin and other cytoskeleton
proteins as a means of breaking free to continue infection, none of them
is known to use actin as a cofactor in this process, Mangel said. "Thus,
the really interesting finding in this case is that actin is a cofactor
for its own destruction," Mangel said.

The next step for Mangel's group will be to crystallize complexes of
actin bound to the adenovirus protease to determine its atomic structure
at the National Synchrotron Light Source at Brookhaven.

"The structure would then be used to find drugs to prevent the
interaction between actin and the adenovirus protease," Mangel said. "Such
drugs could serve as a new type of antiviral agent."

This work was done in collaboration with Mark Brown, Kevin McBride,
Mary Lynn Baniecki, and Nancy Reich of Stony Brook University and Gerard
Marriott of the University of Wisconsin. It was funded by the U.S.
Department of Energy, which supports basic research in a variety of
scientific fields, and the National Institutes of Health.

The
U.S. Department of Energy's Brookhaven National Laboratory conducts
research in the physical, biomedical, and environmental sciences, as
well as in energy technologies. Brookhaven also builds and operates
major facilities available to university, industrial, and government
scientists. The Laboratory is managed by Brookhaven Science
Associates, a limited liability company founded by Stony Brook
University and Battelle, a nonprofit applied science and technology
organization.